(1) Field of the Invention
The present invention relates to an aqueous jet ink composition, and more particularly, to an improved aqueous jet ink composition which is capable of being used for a long period of time without causing clogging in the nozzle portion of a jet printer, and which can provide printed matter having good resistance to light.
(2) Description of the Prior Art
The basic printing process using an ink jet comprises introducing an ink into a fine nozzle composed of a metal or glass (having an inner diameter of about 10 to about 200.mu.), jetting uniform ink particles from the top end of the nozzle and electrostatically deflecting the jetted particles by a deflecting electrode disposed in front of the nozzle to draw a pattern or figure according to a predetermined dot matrix.
Appropriate viscosity, surface tension and electric characteristics can be decided according to methods for jetting inks in the form of uniform particles and controlling the ink particles. In any ink jetting method, however, an ink must be jetted from a fine nozzle in the form of uniform particles in a continuously stable manner.
Accordingly, the basic problem in an ink jet printer is how to completely prevent clogging of the mozzle portion and ensure stable generation of uniform ink particles continuously over a long period of time. Another important requirement is to be able to again form uniform ink particles stably after the jetting operation has been temporarily stopped.
Of course, if a bit of a solvent-insoluble solid adheres to a fine nozzle, clogging naturally wound take place. Furthermore, if the solid adheres to a part of the nozzle, the ink would be jetted in a different direction. Accordingly, in order to perform jetting of ink particles stably and continuously, it is necessary not only to maintain the viscosity and surface tension of the ink composition at appropriate levels, but also to remove solvent-insoluble solids completely from the ink composition. Moreover, it is necessary to prevent occurrence of an undesirable phenomenon wherein solvent-insoluble solids are formed in the ink composition with the passage of time.
Thus, inks that are used for the ink jet printing method for forming letters of patterns on a recording material by jetting liquid drops from a very fine nozzle are required to have the above-mentioned special properties. In other words, it is necessary to use an ink composition which does not contain particles that could cause clogging of the nozzle and which does not become dry or yield precipitates in the nozzle opening, even if the printing device is stopped for a certain period of time.
Various aqueous jet ink compositions comprising a water-soluble dye, a wetting agent and water have heretofore been known as ink compositions meeting the above requirements. For example, there can be mentioned a composition comprising a water-soluble dye, water and a lower alkoxy triglycol, polyethylene glycol, polypropylene glycol or a lower alkyl ether of ethylene glycol, diethylene glycol, propylene glycol or glycerol and having a viscosity of from 1.0 to 10 cp (as measured at 25.degree. C.) and a specific resistance lower than 150 .OMEGA.-cm (see U.S. Pat. No. 3,846,141), a jet ink composition comprising a water-soluble dye, a non-volatile water-soluble polymer oil and a chelating agent (see Japanese Patent Laid-Open No. 85804/76 (U.S. Patent Application Ser. No. 534,520)), a jet ink composition comprising water, a water-soluble dye and a polyhydric alcohol (see Japanese Patent Publication No. 8361/75), and an infrared absorption jet printing ink composition comprising a coloring material having 50% absorption in the wavelength region of 700 to 1000 nm (dye or water-dispersed carbon black), an alkylene glycol or alkyl ether of alkylene glycol and water and having a viscosity of 1 to 10 cp (as measured at 25.degree. C.) and a specific resistance of 50 to 5000 .OMEGA.-cm (see U.S. Pat. No. 3,705,043).
However, these ink compositions comprising water, a water-soluble dye and a wetting agent are defective in that colored letters or patterns formed by ink jet printing are extremely poor in resistance to light and hence will disappear when exposed to the sun light.
As a means to eliminate these defects, there has been proposed that a jet ink composition in which a premetallized dye, that is, a dye complex with a metal such as chromium or copper, is used instead of the water-soluble dye, and a lower aliphatic monohydric alcohol and an inorganic salt are used in combination with such premetallized dye (see U.S. Pat. No. 3,994,736).
However, it is impossible to produce all the hues by such premetallized dyes having good light resistance, and the composition is defective in that high chromatic color cannot be obtained in regions of the three primary colors.
It is known that an inorganic salt such as Glaubers' salt (Na.sub.2 SO.sub.4), KCl, NaCl, LiCl, NH.sub.4 NO.sub.3 or NH.sub.4 Cl is added to adjust the specific resistance of jet ink compositions (see, for example, U.S. Pat. Nos. 3,846,141, 3,705,043, 3,416,153 and 3,596,275).
However, letters or patterns formed by such jet ink composition are very poor in resistance to light. That is, such inorganic salts act only as a specific resistance adjusting agent and do not contribute to increase in the light-resistance.
As will be apparent from the above illustration, no jet ink composition exists which shows excellent light-resistance while using an ordinary water-soluble dye producing high chromatic color.
In an aqueous jet ink composition, coloration is ordinarily accomplished by the use of a water-soluble dye while removing solvent-insoluble solids and preventing formation of such solids.
In an aqueous jet ink composition comprising a water-soluble dye, it is easy to satisfy the basic requirement of the ink jet printing method, that is, the prevention of the formation of solvent-insoluble solids, but there is a defect involved wherein the light resistance of the obtained prints printed matter is insufficient.
More specifically, recorded letters disappear or become difficult to read when a certain period has elapsed. Furthermore, in the case of color images, discoloration or color change is often caused by differences in light resistance among mixed dyes.
In other words, water-soluble dyes customarily used for the conventional aqueous jet ink compositions are poor in light-resistance. Moreover, because of various conditions required to satisfy quality requirements in jet ink compositions, such as prevention of clogging of the nozzle and an increase in the adaptability to continuous jetting and also because of hue and chroma required in prints, the kinds of water-soluble dyes applicable to the ink jetting printing method are naturally limited.
With the foregoing as a background, we conducted research with a view to developing a novel jet ink composition which would not cause clogging of the nozzle, as is often experienced in the conventional compositions, and which would improve the light resistance of prints and permit an increase in the kinds of applicable water-soluble dyes.
As a result of such research, we arrived at the novel jet ink composition of the present invention comprising an aqueous jet ink and an ultraviolet absorbing agent, and if desired with a specific metal salt. When the composition of the present invention is used, formation of solvent-insoluble solids causing clogging of the nozzle can be prevented and the light-resistance of prints can be improved without any reduction in the excellent recording characteristics of the ink jet recording method.